Osteoporosis cannot be reversed nor cured, therefore it needs to be prevented. In fashioning preventive strategies the multifactorial genesis of the condition must be considered. This epidemiological study addresses the issue of the many bone determinants with special emphasis on mild levels of physical activity as a likely moderator of bone loss. The clinical trial, for which this continuation is requested, involves 255 post-menopausal women, 57 years of age at entry. The women were randomized into two groups: the walking and the control groups. Walking was selected as an intervention tool, because it is a mode of activity applicable to the broadest segments of population including the aged and the frail. The women will be followed for five years of annual cycles of assessments, during which morphometric variables, activity levels, nutritional intake will be evaluated; also, measures of cortical bone, determined with a CT scanner at the shaft of the radius, will be taken. The specific aims of the study are: 1) to find whether moderate physical activity, typified by walking, can slow the rate of age-related bone loss (ABL); 2) to define the determinants, physical activity included, of the natural history of ABL and their relative roles in bone changes over time; 3) to attempt, in the final year of the study, to define the profile of the woman at a higher than average risk of developing osteoporosis. To this end "fast" and "slow" losers of bone will have in the final year serum sex hormone levels determined as well as their lumbar trabecular bone measured by Genant's QCT method. The ultimate goal is to make a contribution to a comprehensive package of preventive strategies for osteoporosis.